1. Field of the Invention
The invention relates to a picture display device comprising a display tube having a system of deflection coils, which display tube comprises an evacuated envelope having therein an electron gun for generating at least one electron beam and a display screen, which electron beam is focused on the display screen by means of a focusing lens, over which display screen the electron beam is deflected in two mutually perpendicular directions by means of the system of deflection coils.
Such a device is used for projection television, for displaying monochromatic and colour television pictures and for displaying letters, digits, symbols and figures in one or more colours. Tubes for such devices are available in very many constructions and are manufactured on a very large scale.
2. Description of the Prior Art
Such a device is known inter alia from chapter I of the book "Electron Optics in Television" Pergamon Press, Oxford 1961. In such devices it is known that the spot of the electron beam on the display screen in the centre of said display screen has different dimensions from the spot of the deflected electron beam which is situated at the edge of the display screen. This effect which is the result of curvature of the image field and astigmatism is termed deflection defocusing and leads to less sharp pictures on a part, for example, the edge, of the display screen. This is annoying in particular when letters, digits and symbols are displayed. In colour display tubes having three electron beams convergence problems occur in addition due to said deflection defocusing.
So-called self-converging systems of deflection coils have been designed in which, as described in U.S. Pat. No. 2,866,125 and which is incorporated by reference in this Application, a ribbon-shaped electron beam remains focused on the display screen during the deflection. In colour display tubes said ribbon-shaped electron beam is in practice often formed by three sub-electron beams situated in one plane. This beam must be ribbon-shaped, in other words, must have a small dimension in one direction, because otherwise upon deflection extra deflection focusing occurs. A property of such a system is that in the direction of the plane of the ribbon the focusing, that is the adjustment of the focusing lens in which the spot in that direction has a minimum dimension, is substantially not dependent on the deflection.
In practice the electron beam emanating from one electron gun has by no means an infinitely small cross-section and often is circular. The problem of the deflection defocusing is still present "microscopically" (hence considered per electron beam) in the direction perpendicular to the plane of the ribbon in such self-converging deflection coil systems. Dynamic focusing does not provide a solution to this problem because dynamic focusing in one direction automatically involves defocusing in the other direction.
It is therefore an object of the invention to provide a solution to this problem and to provide a device in which the focusing in two mutually perpendicular directions is independent.
Another object of the invention is to provide a device in which it is possible in a comparatively simple manner to reduce the spherical aberration of the electron beam.